The present invention relates to methods of making optical quality coatings of inorganic oxides on glass or equivalent substrates, such as metals or ceramics. These inorganic oxide coatings are electrochromic and have other desirable properties which allow their use as electrically conducting coatings, optical coatings, wear resistant coatings and anti-reflective coatings. Electrochromic films undergo reversible coloration induced by an applied electric field or current. These inorganic electrochromic layers can be broadly classified into those that color cathodically due to the double injection of electron and cation (Group VI-B oxides such as WO.sub.3 and MoO.sub.3) and those Group VIII oxides that color anodically (such as IrO.sub.2, Rh.sub.2 O.sub.3, NiO and CoO). Such coatings are used in information display devices, solar control windows and light modulators.
The most common way to deposit electrochromic films is by vacuum techniques, typically by evaporation and sputtering. Non-vacuum techniques such as anodization and atmospheric chemical vapor deposition are also reported. Evaporation deposition and sputter coating require a high vacuum. While such techniques require expensive capital equipment, they have been commonly used to produce electrochromic coatings.
Three similar non-vacuum coating techniques which have been used to a limited extent for electrochromic coatings are dip coating, spray coating and spin coating. These techniques offer the advantage of being less capital intensive and thus less expensive. Dip coating, as an example, is commonly used to coat glass with SiO.sub.2. This process involves lowering a glass substrate into a solution containing an appropriate precursor of the desired oxide. Spin coating and spray coating are similar to dip coating except that instead of dipping the glass, the precursor solution is applied to the glass, which is spun to spread the coating out, or is sprayed onto heated glass. For all of these methods, the typical starting solution consists of an organic metallic compound which is complexed in a suitable solvent, e.g., for SiO.sub.2 coating, tetraethylorthosilicate is dissolved in an ethanolic solution and partially hydrolyzed. In the dipping method, the substrate (glass) is dipped into the solution and withdrawn at a known rate (usually several centimeters per minute), thereby coating the surface with solution. The coating is then dried and fired in an oven to complete hydrolysis and condensation and to densify the newly formed oxide coating.
However, some of the starting materials of the important electrochromic materials, such as alkoxylates, are expensive and do not afford satisfactory coating results if directly dissolved in typical solvents. Also, many commercial dip coatings and commercial processes involve preparation of the dip coating solution in bulk and its subsequent use over several days of dip coating. Such solutions of precursors typically have a gestation period during which hydrolysis is occurring in solution to form a stage of partial hydrolysis suitable to form an acceptable coating. This is followed by a period over which the solution can be used for dip coating. Beyond this window of use, the viscosity of the solution rises such that the solution once again becomes unsuitable for dip coating, and is typically then discarded. Hence, an important low cost approach to creating electrochromic coatings appears to be impractical based on these materials.
French Patent No. 2,527,219 discloses dipping in a colloidal polymetallic acid of a transition metal preferably in aqueous medium. However, such suspensions are reported to be very unstable, having a useful life of 24 hours or less. Nakatani et al., U.S. Pat. No. 4,420,500, disclose the deposition of transparent conducting films onto glass and ceramic substrates from a coating composition containing indium compounds and alkyl tin nitrate compounds. Moser et al., U.S. Pat. No. 4,959,247, disclose the dipping in a solution of transition metal nitrates dissolved in monoalcohols having from 1 to 5 carbon atoms. The coatings are applied to the substrate by a dipping method. However, when using this method, the dipping solution has a limited window of use before and after which desired quality coatings are not formed.
In a similar patent, Moser et al., U.S. Pat. No. 4,855,161, the disclosure of which is herein incorporated by reference, disclose a method for coating a substrate with electrochromic coatings by dipping a substrate having a conductive surface into, or spraying, or spin coating with a solution prepared by reacting an anhydrous halide of an electrochromically active transition metal with a C.sub.1 to C.sub.5 anhydrous alcohol. However, in this method it is difficult to expel halides at low firing temperatures.